Various manufacturing processes involve the transmission of a heated liquid from a supply tank, through a hose, and to a liquid dispensing device which deposits the heated liquid into a container or onto a substrate. Some of the heated liquids are hot melt adhesives which solidify at room temperature. Accordingly, a hot melt adhesive must be heated and liquified so it can flow from the supply tank, through the hose, and out the liquid dispensing device. To liquify and subsequently maintain the hot melt adhesive within an appropriate temperature range, the supply tank, the hose, and the dispensing gun are selectively heated by individual heating devices operatively associated with each respective component. To monitor the temperature of the hot melt adhesive throughout the application process, each component further includes some form of temperature sensing device which operates in conjunction with at least one heating device. A controller operates the heating device in response to signals from the temperature sensing device to maintain the hot melt adhesive within a predetermined temperature range.
Generally, separate temperature controllers are provided for the dispensing gun, the hose, and the supply tank. The hose will often incorporate a single temperature sensing device, such as an RTD, and a single heating device which are coupled to a wire harness extending from one end of the hose. This wire harness has a connector which connects to a complementary connector on the controller. The controller monitors the temperature detected from the RTD and activates the heating device as necessary. The RTD may be made from different materials, such as nickel or platinum. Typically, either a nickel RTD is used with a compatible controller, or a platinum RTD is used with a different compatible controller. U.S. patent application Ser. No. 09/697,572 filed Oct. 26, 2000, and assigned to the assignee of the present invention, discloses the incorporation of both a nickel RTD and a platinum RTD into a heated hose. Through the use of an adaptor plug, this allows the hose to be operatively coupled to either of the two types of controllers in use (i.e., platinum or nickel RTD compatible controllers).
Occasionally, temperature sensing devices such as RTDs, will fail or otherwise malfunction. This leads to erroneous temperature readings or to a complete inability to detect the temperature of the intended target, such as the liquid adhesive being carried within a heated device such as a hose. In these cases, since the RTD is integrally incorporated into the heated device, the entire heated device must be disassembled from its associated system and replaced. The downtime and replacement costs can be relatively high, especially as compared to the cost of the RTD itself. It may also be some time before a defective RTD is discovered and this can result in improper heating of the adhesive for the same amount of time. If overheating of adhesive occurs, char and other negative effects of the overheating can harm the hot melt system and/or the products receiving the hot melt adhesive. Underheating the adhesive can, for example, adversely affect adhesive properties such as bond strength.
In light of the drawbacks discussed above, it would be desirable to provide a heated device for carrying a liquid in which the heated device can automatically respond to a temperature sensor failure and/or which has redundant temperature sensing capabilities. These capabilities would provide for accurate temperature sensing in the event of sensor failure and provide for easier and less costly maintenance of the heated device.